Future Fifth Generation (5G) wireless systems will need large antenna gains—which is achieved by having multiple antennas—in order to compensate for the very strong path loss of the millimeter-wave (mmWave) frequencies. There are even some discussions about massive Multiple-Input-Multiple-Output (MIMO) (i.e., Massive-MIMO) transmitters which would have hundreds of transmit antennas, and therefore, hundreds of radios.
In order to have energy efficient architectures, the Peak-to-Average Ratios (PARs) that each power amplifier will have to support has to be significantly lower than what they are today (typically around 7 decibels (dBs) at baseband). This is for two reasons, namely, (1) power amplifiers are much more efficient at low PARs and (2) it will not be possible to perform Crest Factor Reduction (CFR) as well as power amplifier predistortion on hundreds of power amplifiers and be energy efficient at the same time. Thus, there is a need to eliminate or reduce the need for both CFR and predistortion in the radio.
More specifically, FIG. 1 illustrates a conventional cellular transmitter 10 for an Orthogonal Frequency Division Multiplexing (OFDM) based system (e.g., a Long Term Evolution (LTE) network). As illustrated, the transmitter 10 includes an OFDM modulator 12, which includes a Serial-to-Parallel (S/P) converter 14 that converts a serial input data signal into multiple parallel input data signals. Each of the parallel input data signals corresponds to a different OFDM subcarrier. The parallel input signals are input to an Inverse Fast Fourier Transform (IFFT) function 16. The IFFT function 16 produces a modulated signal. A Cyclic Prefix (CP) function 18 inserts a cyclic prefix, as will be appreciated by one of ordinary skill in the art. The modulated signal output by the OFDM modulator 12 is provided to a radio front-end of the transmitter 10. Note that the “cloud” illustrated in the figure is to show that there may be additional components (e.g., filter(s), cables (e.g., an optical cable for a Common Public Radio Interface (CPRI) link), and/or the like) between the OFDM modulator 12 and the radio front-end of the transmitter 10. The radio front-end includes a CFR function 20 that perform CFR according to some CFR scheme. The output of the CFR function 20 is then predistorted by a Digital Predistortion (DPD) function 22. The predistorted signal is upconverted by an unconverter 24. At some point, either prior to, during, or after upconversion, the signal is converted from digital to analog. The upconverted, analog signal is amplified by a Power Amplifier (PA) 26 and transmitted via an antenna 28. The radio front-end also includes a Transmitter Observation Receiver (TOR) that includes a downconverter 30 having an input that is coupled to the output of the PA 26 via a coupler 32. The transmit signal is downconverted by the downconverter 30 to provide an observed transmit signal. An adaptor 34 then adapts the predistortion applied by the DPD function 22 according to some adaptation scheme (e.g., to compensate for a non-linear characteristic of the PA 26).
If the architecture of the conventional transmitter 10 were scaled up to support Massive-MIMO, the transmitter 10 would then include many (e.g., up to hundreds) of the radio front-ends. Therefore, the efficiency of the PA 26 becomes extremely important. Further, having hundreds of CFRs 20, DPDs 22, and TORs results in a large amount of complexity and power consumption. Thus, two main challenges for 5G Massive-MIMO are PA efficiency and transmitter complexity.
For 5G Massive-MIMO transmitters, it would be desirable to have extremely low PARs for each of the PAs (e.g., in the order of 3-4 dBs maximum) in order to get very good power efficiency. In addition, it would be desirable to eliminate or at least reduce the complexity of the CFR, DPD, and TOR in the radio front-end.